There is a trend in the United States in recent years in mandating that a tire air pressure monitoring system for monitoring tire air pressure be mounted in automobiles. Conventionally, there are systems that do not require a battery because of the use of a surface acoustic wave (SAW) device as a method for directly measuring the air pressure of a tire.
An RFID (Radio Frequency Identification) technique is known in which a wireless connection is provided between a tire-mounted pressure sensor and a controller on the vehicle body, and the two are electromagnetically coupled together.
Patent document 1 noted below describes a technique whereby electromagnetic waves are sent from a controller toward a sensor in which the resonance frequency varies in accordance with a measured value, and the measured value is determined by the controller on the basis of variations in the frequency of the electromagnetic waves returned from the sensor.
[Patent Document 1] U.S. Pat. No. 6,378,360